This present invention concerns a method and a device for automatic determination of the permeability of an object of porous material, where this object can come in the form of a sheet or a strip of porous material consisting of successive segments which alternately present different levels of permeability according to a pre-established periodicity.
It applies particularly, though not exclusively, to monitoring and measurement of the permeability of the segments of a paper strip of the type such as described in patent EP 0486213 A1 with cellulose type fiber weave, which consists of a succession of transverse segments of different densities and therefore of different permeability. This paper strip can be used, for example, for the production of self-extinguishing cigarettes.
In general, it is known that the permeability of a sheet of porous material can be ascertained by means of a permeability meter consisting of a measuring head with two tubular parts that move in relation to each other, in such a way as to butt up against each other in order to grasp the sheet, thereby respectively delimiting two coaxial chambers on two distinct faces of the material. One of these chambers is connected to a measurement circuit consisting of a flowmeter, pumping resources capable of creating a pressure or a vacuum in the measurement circuit, and regulation resources capable of maintaining a determined pressure in the measurement circuit.
The manufacturers of strips of porous material of the aforementioned type, and also the users of such strips, wish to be able to monitor the permeability of the porous segments of the strip by means of a measurement at the center of each strip, regardless of whether this is a segment of low permeability or a segment of high permeability.
In order to achieve such results, the following two conditions must be satisfied:
the use of a measuring head whose chambers are of smaller dimensions than those of the segments, so that when the two chambers are centered on a segment, the measurement will be effected only at a central zone of this segment, and
implementation of the method and device which allow the measuring head to be centered on each of the segments whose permeability is to be measured.
The first of these conditions is easily satisfied if the dimensions of the segments are known. In practice, these will be supplied by the paper manufacturers.
However, the second condition gives rise to a significant problem because of the fact that the porous segments of the strips are not visible, and so it is not possible to initialize the position of the strip by a visual reference.